Home     Gallery     Field Guide     Diving     Journals     Sponsors/Thank Yous  


ANCHOR ICE


Anchor ice .... clear jagged plate-like ice carpeting the seafloor in crystalline sheets and fastened to submerged objects like rocks, gravel, and animals. Anchor ice forms on the seafloor at depths shallower than 33 meters, and most significantly at depths shallower than 15 meters. Under the sea ice, anchor ice plays a key role in the shallow benthic ecosystem as a natural physical disturbance.


In deeper water seawater cools below the surface freezing point. When the seawater is upwelled to shallower water and pressure is reduced, it drops below the in situ freezing point and anchor ice freezes out. However this does not fully explain such a well-defined lower limit of ice formation, and the shoaling and freezing of supercooled deep water originating below the ice shelf may be at work. When anchor ice grows large enough to become buoyant and break free, chunks of it float slowly up and join the platelet ice on the undersurface of the sea ice ceiling.

Anchor ice forms wherever conditions are suitable and that may be on or around a slow-moving or sessile organism. Anchor ice can entrap sea urchins, seastars, worms, isopods, sea spiders, sponges, and other animals as well as algae, rocks, and sediment. Animals may remain alive surrounded by an ice prison, or become frozen and killed. When an anchor ice mass becomes bouyant, it lifts off the bottom carrying its entrapped organisms up to the sea ice ceiling. Some become a food source for animals and bacteria under the sea ice ceiling. Some get frozen into the sea ice ceiling or may rain back down to the bottom some distance from where they floated up. Anchor ice also serves as a refuge from predators with its numerous small cracks, crevices, and caves.

1: Polar Biology 8:377-391, 1988; 2: Science 163(3864):273-274, 1969; 3: Science 167(3915):171-172, 1970; 4: Deep Sea Research 21(3):169-174, 1974

Text ©Peter Brueggeman. Photographs ©Norbert Wu. Photographs may not be used in any form without the express written permission of Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?